FIG. 1 is a block diagram illustrating a MIMO (Multiple Input Multiple Output) system. The MIMO system shown in FIG. 1 performs signal transmission/reception using a plurality of antennas, such that it can transmit desired data at high speed. Provided that the number of antennas of a base station is set to “M”, the number of terminal antennas is set to “N”, and the number “M” is higher than the number “N”, capacity of a downlink MIMO channel is proportional to the number “N”, the base station simultaneously transmits N data streams, resulting in the implementation of high-speed data transmission.
In the meantime, an automatic repeat request (ARQ) method controls a reception end to determine the presence or absence of erroneous transmission packets, transmits the determined result to a transmission end, and retransmits the erroneous packet when the presence of the erroneous transmission packets is determined. If the presence or absence of the erroneous packets is fed back to the reception end, the ARQ method transmits an acknowledgement (ACK) signal to the reception end when the absence of the erroneous packets is determined, and transmits a negative acknowledgement (NACK) signal to the reception end when the presence of the erroneous packets is determined.
A Hybrid Automatic Repeat Request (HARQ) method is implemented by applying a channel coding method to the ARQ method. In more detail, although an unexpected erroneous packet is detected from among received packets, the HARQ method does not discard the erroneous packet, it combines the erroneous packets with retransmitted packets, and decodes the combined result, such that it increases a diversity gain and a coding gain. In the meantime, the ARQ method transmits ACK/NACK signals using a high-layer signaling process, resulting in the occurrence of an increased delay. The HARQ transmits the ACK/NACK signals using a physical-layer signaling process, resulting in a reduced delay. When packets are transmitted via a plurality of antennas according to the above-mentioned HARQ method, the above-mentioned HARQ scheme has difficulty in determining how to apply the space-time coding method to retransmission packets.